Rotary pumps for viscous fluids



Aug. 13, 1968 A. SCHMITT 3,396,667

ROTARY PUMPS FOR VISCOUS FLUIDS Filed Oct. 24, 1966 2 Sheets-Sheet 1 Fig. 1

IN VEN TOR.

ktmnv QcHMITT BY 2 a INTO/1 5 3 Aug. 13, 1968 A. SCHMITT ROTARY PUMPS FOR VISCOUS FLUIDS 2 Sheets-Sheet 2 Filed Oct. 24, 1966 INVENTOR.

mamu sum/Tr United States Patent 3,396,667 ROTARY PUMPS FOR VISQOUS FLUIDS Armin Schmitt, Kaiserslautern, Germany, assignor to Eisenwerlre Kaiserslautern G.rn.h.H., Kaiserslautern, Germany Filed Oct. 24, 1966, Ser. No. 588,986 Claims priority, application Germany, Oct. 29, 1965 E 39,386 5 Claims. (Cl. 103-126) The present invention relates to a rotary pump of the Roots type for conveying viscous fluids and suspensions which comprises a pair of interengaging rotors forming pump vanes which are mounted on and are driven by a pair of parallel shafts at equal speeds in opposite directions within an oval housing which is provided with an intake duct and an outlet duct, and in which the pump vanes of the two rotors which are angularly offset relative to each other rotate along the inner wall of the housing in sliding engagement therewith and also roll along each other so that, while one rotor carries out its pumping function, a pump vane of the other rotor acts as a sealing element upon the first rotor. For driving the two rotors at equal speeds in opposite directions, their shafts are usually connected by a pair of meshing gears. The working surfaces of these rotor vanes of all Rootstype pumps of the known types have a curved cross section which differs in shape in pumps of different constructions. These working surfaces of all such pumps have, however, in common that their outer ends extend at an acute angle to the inner wall of the housing on which they are in sliding engagement.

These conventional Roots-type pumps are usually made of cast iron, gun metal, zinc-free bronze, or highgrade steel. In selecting the proper materials for the pump housing and the rotors, respectively, it is, of course necessary to insure that the contact surfaces of the two different kinds of parts will have adequate gliding properties to prevent the rotor vanes from seizing. Therefore, the housing and the lateral bearing covers of such pumps, especially if they are intended for conveying highly viscous fluids or suspensions, are usually made of chrome-plated cast iron, while the rotor vanes and their drive shafts are made of hardened high-grade steel.

The conventional Roots-type pumps as above described have a series of disadvantages:

Since the proper function of such a pump requires a sufficient clearance between the pump housing and the rotor vanes and also between each vane and the core of the adjacent rotor, a certain amount of the fluid to be conveyed will flow back from the delivery side to the intake side of the pump. Especially when conveying highly viscous fluids or suspensions which have a corrosive influence, the clearance between the pump housing and the rotor vanes or between the latter and the rotor cores will increase after the pump has been in operation for a relatively short time. This, in turn, will result in a considerable increase of the back flow of the fluid within the pump and thus in a considerable reduction of the efliciency of the pump. Furthermore, especially if the difference in pressure between the intake and delivery sides of the pump is very high, erosion may occur on the surfaces of the vanes which roll along each other. Such erosion will further increase the normal wear on the movable parts of the pump. A replacement of these parts when they have become useless because of normal wear, corrosion and erosion is extremely expensive especially in view of the high-grade materials of which they are made.

It is an object of the present invention to provide a rotary pump of the Roots type which, although especially adapted for conveying viscous fluids with corrosive properties, may be made of materials of a lower cost than 3,396,667 Patented Aug. 13, 1968 those which were previously required for producing rotary pumps of this type, and which permits those parts of the pump which are especially subjected to wear to be replaced very easily and also at a relatively low cost.

For attaining this object, the invention provides that the pump housing of the usual oval cross section may be made of any suitable mechanically resistant material and by providing it with a layer of enamel on the inner surfaces, while the rotors of the pump are made of a noncorrosive and wear-resistant plastic and are rigidly secured to drive shafts which may also serve as rotor cores. Another feature of the invention consists in designing each of the two rotors so as to form a single pump vane of a segmentally-shaped cross section, the peripheral surface of which is adapted to slide along the semicircular inner surfaces of the pump housing while the substantially radial surfaces of each rotor serve as working surfaces and are made of a substantially S-shaped cross section. These working surfaces are adapted to slide or roll with a wiping motion along each other and each of them merges gradually in the form of a continuous curve with the outer peripheral surface of the rotor, while its inner end extends at an obtuse angle, for example, tangentially, to the peripheral surface of the rotor core which may be formed by the shaft of the rotor.

The above-mentioned and additional features of the invention in combination with each other results in a series of very important and valuable advantages. While prior to this invention it was considered necessary to make the pump housing and the rotor vanes of very expensive materials, it is now possible to produce these parts very simply and of very inexpensive materials. Although enamel has for ages been employed in many different arts for providing metals with an extremely resistant surface coating, its use in rotary pumps has never been considered. Such a hard enamel coating on the inside of the housing of a rotary pump permits this housing to be made of an easily corrosive material, for example, unalloyed steel or ordinary cast iron. The inventive association of materials between the hard enamel coating on the housing and a suitable plastic of which the rotors are made also insures that there will be practically no mechanical wear upon the enamel coating of the housing since the plastic rotors have sufiicient resilience so as to yield when small solid particles are contained within the fluid to be conveyed. Any mechanical wear which will occur in the course of time will therefore be confined only to the rotors which may then be replaced at a very low cost when the efliciency of the pump decreases.

The new association of materials, that is, between the hard enamel coating on the inner wall of the housing and the relatively soft plastic of the rotors requires, however, the rotors to be made of a particular design in accordance with the properties of these materials. This is attained according to the invention by making the rotors of the simplest possible construction without any rectangular or acute edges which might be shorn off by the shearing forces occurring in the operation of the pump and which when severely damaged would render the rotors useless prematurely.

By providing the pump according to the invention with an internally enameled housing and a pair of pump rotors, each of whidh consists of plastic, it is now possible to employ rotary Roots-type pumps for also conveying highly corrosive fluids or suspensions of a high viscosity very economically and without requiring any replacement of the pumps or even of their rotary parts for a very long time. Another very important advantage of the pump according to the invention over other Roots-type pumps is attained by providing each pump rotor in the form of only one vane and by designing this vane of a particular shape.

These single-vane rotors alternately result in pump chambers of a very large size and thus produce a very high output of the pump even atga low driving speed. Such a low driving speed, in turn, results in less wear upon the pump which will thus operate efliciently for a very long time. The further feature of the invention of making the Working surfaces of the rotors of a particular crosssectional shape, namely, substantially S-shaped, has the ad vantage that a viscous fluid will be conveyed practically without being squeezed or compressed.

Although various kinds of plastic may be employed for producing the rotors, they are preferably made of polytetrafluor ethylene. The excellent chemical stability and the other technological properties of this material insure that the wear upon the rotors will always be low and that the efliciency of the pump will remain nearly constant for a long period of time.

For eliminating the dangerous erosion which may occur on the working surfaces of the rotors if in the operation of the pump there is a high difference in pressure between the intake and delivery side of the pump, it is another important feature of the invention to provide these working surfaces with shallow recesses which are designed so that, when a working surface of one rotor engages with one of the other rotor, the recesses in the two surfaces will slightly overlap each other. Thus, when the adjacent working surfaces of the two rotors roll along each other within the space between the cores of the two rotors, a narrow slot is formed by the partly overlapping recesses which permits the squeezed fluid to flow off toward one side.

Another feature of the invention consists in making the rotors of a segmentally-shaped cross section and the angle in the segment of one rotor slightly smaller than 180 and the angle in the segment of the other rotor slightly larger than that of the first rotor and nearly 180.

These and further features and advantages of the present invention will become more clearly apparent from the following detailed description thereof which is to be read with reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 shows a longitudinal view of one of the pump rotors according to the invention, as seen in the direction toward its working surfaces;

FIGURE 2 shows a cross section which is taken along the line IIII of FIGURE 1; while FIGURES 3 to 6 show similar cross sections of the pump housing with the two pump rotors in four different stages in the operation of the pump.

As illustrated in the drawings, the rotary pump according to the invention essentially consists of an oval housing 1 and a pair of rotors 2. Housing 1 may, for example, be a casting of cast iron or a similar material and it is coated on the inside with an enamel layer 11. Together with its lateral semicircular projections, housing 1 forms a pair of pump chambers A and B for the operation of the rotors 2. These pump chambers are connected at the lower end of housing 1 to an intake duct 12' and at the upper end of the housing to a delivery or outlet duct 13.

Each rotor 2 consists of a solid body 21 of plastic forming a single pump vane, and a core 22a which is mounted on or forms the end of a drive shaft 22 which is rotatably mounted in the housing 1. At the outside of the housing, the two drive shafts 22 are connected to each other by a pair of meshing gears, not shown, so as to be rotatable at the same speed in opposite directions. The two rotor bodies 21 which are made of a relatively hard plastic, for example, polytetrafluor ethylene, have a segmentallyshaped cross section, as seen within the plane as illustrated, and the angle within the segment of one rotor body 21 is slightly smaller than 180, while that of the other rotor body amounts approximately to 180?. In its longitudinal direction, each rotor body 21 has a rectangular cross section and its outer edges are rounded substantially in accordance with the rounded edges of the enamel layer 11 on the housing 1. The rotor bodies 21 are rigidly secured to the rotor cores 22a by means of screws 23 with countersunk heads and by splines 24.

The two working surfaces 25 of each pump rotor 21 are provided with shallow rectangular recesses 26, and when during the rotation ofthe rotors their working surfaces 25 are in engagement with and roll along each other, as shown in FIGURES 3 and 5, the adjacent recesses 26 will partly overlap each other so that a small amount of the fluid to be conveyed may flow back freely toward the intake duct 12. Each of these surfaces 25 has a substantially S-rshaped curvature, as seen in a direction at right angles to the direction of flow, and each of them merges toward the outside by a continuous curvature with the outer peripheral surface of the rotor body 21, while at the inside it is connected at an obtuse angle, for example, tangentially, to the rotor core 22a.

The mode of operation of the rotary pump according to the invention as illustrated in FIGURES 3 to 6 is as follows:

The two rotors 2 and 2' which are geared together so as to be rotatable at the same speed in opposite directions divide the inside of the housing 1 into the two pump chambers A and B. In FIGURE 3, rotor 2 carries out a delivery stroke and rotor 2 carries out an intake stroke, while the space between the two rotor cores is sealed toward the intake and outlet ducts 12 and 13 by the working surfaces 25 of the two rotor bodies 21 rolling along each other. FIGURE 4 illustrates the positions of the rotors 2 and 2 after turning relative to the position as shown in FIGURE 3 at an angle of approximately in the clockwise and counterclockwise direction, respectively. In this stage of the operation, rotor 2' has started a delivery stroke and piston 2 has started an intake stroke. In the next stage, as shown in FIGURE 5, in which the two rotors are turned at an angle of approximately relative to the position as shown in FIGURE 3, rotor 2 has reached approximately the middle of its intake stroke, while rotor 2' has reached approximately the middle of its delivery stroke. In FIGURE 6, the two rotors 2 and 2 have reached a position in which they are turned at an angle of approximately 90 relative to the position as shown in FIGURE 5 and rotor 2' just starts to carry out an intake stroke and rotor 2 a delivery stroke. In FIG- URE 6 it may also be seen that the volume of the pump chamber A which is formed between body 21 of rotor 2 and the wall of housing 1 substantially corresponds to the maximum volume of fluid which is to be conveyed by rotor 2 during one complete revolution thereof. This, of course, also applies to pump chamber B and the rotor 2' when both rotors are further turned for one half revolution in opposite directions.

When rotors 2 and 2 are further turned at an angle of about 45 relative to the position as shown in FIGURE 6, they will again reach the operating stage as illustrated in FIGURE 3.

While the two rotors are in the positions as illustrated in FIGURES 4 and 6, the free space between the two rotor cores 22:: will be sealed by the body 21 of one rotor rolling along the core 22a of the other rotor. There will then be a linear sealing engagement between the outer peripheral surface of one rotor body 21 and the free surface of the core 22a of the other rotor.

Although my invention has been illustrated and described with reference to the preferred embodiment thereof, I wish to have it understood that it is no way limited to the details of such embodiment but is capable of numerous modifications within the scope of the appended claims.

Having thus fully disclosed my invention, what I claim is:

1. A rotary pump for conveying viscous fluids or suspensions comprising an oval housing having an intake duct and an outlet duct, an enamel coating on the inner wall of said housing, a pair of parallel shafts rotatably mounted in said housing and laterally spaced from each other and adapted to be driven at the same speed in opposite directions, and apair of rotors consisting of a noncorrosive and wear-resistant plastic secured to said shafts within said housing and serving as pump vanes, said shafts also serving as cores of said rotors, each of said rotors having a segment-shaped cross section and a pair of longitudinal working surfaces determining the angle of the segment, each of said working surfaces having a substantially S-shaped cross section merging by a continuous curve with the outer peripheral surface of said rotor and substantially tangentially with said core.

2. A rotary pump as defined in claim 1, in which said rotors consist of polytetrafluor ethylene.

3. A rotary pump as defined in claim 1, in which said working surfaces are provided with shallow recesses adapted slightly to overlap each other when a working surface of one of said rotors is in engagement with a working surface of the other rotor.

4. A rotary pump as defined in claim 1, in which one of said rotors has an angle within the segment of less than 5. A rotary pump as defined in claim 1, further comprising means for securing each of said rotors to the associated core, said means comprising screws extending in 'diiferent radial directions through said rotor into said core, and a spline connecting said rotor with said core intermediate said screws and extending in the axial direction of said rotor and said core.

References Cited FRED C. MATTERN, JR., Primary Examiner.

180 and the other rotor has an angle within the segment WILBUR J. GOODLIN, Assistant Examiner.

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1. A ROTARY PUMP FOR CONVEYING VISCOUS FLUIDS OR SUSPENSIONS COMPRISING AN OVAL HOUSING HAVING AN INTAKE DUCT AND AN OUTLET DUCT, AN ENAMEL COATING ON THE INNER WALL OF SAID HOUSING, A PAIR OF PARALLEL SHAFTS ROTATABLY MOUNTED IN SAID HOUSING AND LATERALLY SPACED FROM EACH OTHER AND ADAPTED TO BE DRIVEN AT THE SAME SPEED IN OPPOSITE DIRECTIONS, AND A PAIR OF ROTORS CONSISTING OF A NONCORROSIVE AND WEAR-RESISTANT PLASTIC SECURED TO SAID SHAFTS WITHIN SAID HOUSING AND SERVING AS PUMP VANES, SAID SHAFTS ALSO SERVING AS CORES OF SAID ROTORS, EACH OF SAID ROTORS HAVING A SEGMENT-SHAPED CROSS SECTION AND A PAIR OF LONGITUDINAL WORKING SURFACES DETERMINING THE ANGLE OF THE SEGMENT, EACH OF SAID WORKING SURFACES HAVING A SUBSTANTIALLY S-SHAPED CROSS SECTION MERGING BY A CONTINUOUS CURVE WITH THE OUTER PERIPHERAL SURFACE OF SAID ROTOR AND SUBSTANTIALLY TANGENTIALLY WITH SAID CORE. 